The present invention relates to an aromatic polyimide separation membrane prepared by fluorination of an aromatic polyimide membrane with active fluorine. The aromatic polyimide separation membrane according to the present invention has an extremely high selectivity and is useful as a membrane for separation of gas mixtures or liquid mixtures.
Japanese Patent Application Laid-Open (KOKAI) Nos. 57-15819/1982, 58-3603/1983 and 58-8512/1983 describe gas separation membranes prepared from a polyimide obtained from a biphenyltetracarboxylic acid component and an aromatic diamine component. However, although the selectivity of hydrogen against carbon monoxide is relatively high, the permeation rate of hydrogen is not sufficiently high. Japanese Patent Publication (KOKOKU) No. 55-41802/1980 and Japanese Patent Application Laid-Open (KOKAI) Nos. 63-111921/1988 and 63-123420/1988 describe polyimide separation membranes having high gas permeation rates, however, the membranes disclosed in these documents cannot be said sufficient in separation efficiency. Japanese Patent Application Laid-Open Nos. 62-155919/1987 and 63-171619/1988 describe separation membranes prepared by treating substituted polyacetylenes with fluorine, and Japanese Patent Application Laid-Open No. 64-22320/1989 describes a treatment of silicone-based membranes with fluorine, however, the separation membranes of these documents are inferior in durability such as heat resistance and solvent resistance.
The present inventors have carried out extensive research on a separation membrane which shows a higher selectivity than known separation membranes and satisfies the requirements for a useful separation membrane, i.e., a separation membrane excellent in heat resistance, solvent resistance, stability in membrane characteristics and other properties required. As a result, the present inventors have found that a membrane prepared by fluorinating an aromatic polyimide membrane with active fluorine shows an extremely high performance as a separation membrane. The present invention has been accomplished based on this finding.
The modification of polyolefin porous membrane by fluorine treatment has been known in the art, however, in this technique, it has been very difficult to avoid the deterioration in membrane properties due to fluorine treatment. On the contrary, in the present invention, an aromatic polyimide membrane is subjected to fluorination without causing such deterioration. This makes it possible to carry out the fluorination under a wide range of conditions to secure the advantage in practice. It has been also found that an aromatic polyimide membrane subjected to fluorination is improved especially in selectivity.
Further, an aromatic polyimide membrane having a hydrogen permeability coefficient larger than a specific level is found to be effectively fluorinated due to large diffusion rate of active fluorine into the membrane, and therefore, the separation membrane of the present invention can be advantageously obtained by using such an aromatic polyimide membrane.
The separation membrane according to the present invention has a good durability in addition to a high selectivity and a high gas permeability, and is highly useful for practical use.